How can I trim a face thats overlapping another face?
I'm not sure what you are asking but I'll try. you can pull back the part that is sticking out in edit mode. Select the 2 vertices hanging out in edit mode with vertex select checked. Press g to grab and x, y, or z to move along an axis press esc if it goes the wrong way and try again. You can use snap tools to get the mesh to connect properly and not overlap- google: blender snap. I don't think you can just delete the part hanging over unless: there are vertices where a and b intersects then just delete the face hanging over by selecting it and hitting the delete key ( I don't think that is your case though if it was just pulled to far when extruding). alternatively you can delete B, insert a loop cut on face a and extrude from that with the edge selected and snap to what you need to. that might not work to well though unless you are aiming to extrude from the dead center of of face a. after deleting b and inserting a loop cut you could also select the four vertices you want to fill and press f. Hope that helps I didn't understand the problem with C but it might cover that too.